The hu-PBL-SCID mouse model can be a useful tool for assessing the efficiency of certain anti-HIV approaches and for looking at patterns of viral evolution in the face of selection pressure. For instance we have used the model to study viral escape from treatment with cocktails of neutralizing human monoclonal antibodies. Here we propose to use the model for in vivo evaluation of selected entry-blocking strategies and potential subsequent viral evasion. We propose three specific aims: (1) to evaluate in vivo the anti-viral efficiency of a potent human polyclonal neutralizing antibody response by passive transfer of this response to hu-PBL-SCID mice. We will generate in vivo serum escape mutants in order to better understand the specificities of the Abs responsible for the strong serum neutralizing activity. This should help in the design of vaccines capable of inducing an efficient humoral response. (2) to evaluate the role of cell-to-cell transmission in viral replication in vivo. Our results show that in the hu-PBL-SCID mouse, as has been suggested in HIV-1 infected individuals, viral cell-to-cell spread occurring in lymphoid tissue is of primary importance. We will study cell-to-cell spread in the mouse model in order to understand whether cell-to-cell transmission could impair anti-viral strategies based on blocking viral entry. (3) to evaluate the anti-viral efficiency of blocking of HIV-1 coreceptors in vivo. Our preliminary results indicate that Ab-blocking of CCR5 in the hu-PBL-SCID mouse temporarily decreases HIV-1 replication in vivo but may induce the virus to escape coreceptor blocking. We will continue this study and investigate whether blocking coreceptor with Ab cocktails could prevent viral escape. The studies will reveal how HIV-1 can evade entry-blocking strategies, and may provide important information for vaccine design.